Various thermal transfer methods have hitherto been known in the art. In these thermal transfer methods, a thermal transfer sheet comprising a color transfer layer provided on a substrate sheet is image-wise heated from its backside, for example, by means of a thermal head to thermally transfer the color transfer layer onto the surface of a thermal transfer image-receiving sheet, thereby forming an image. The thermal transfer methods are roughly classified according to the construction of the color transfer layer into two methods, i.e., sublimation dye thermal transfer (sublimation-type thermal transfer) and thermal ink transfer (hot melt-type thermal transfer). For both the methods, full-color images can be formed. For example, a thermal transfer sheet comprising layers of three colors of yellow, magenta, and cyan or optionally four colors of yellow, magenta, cyan, and black is provided, and images of the individual colors are thermally transferred in a superimposition manner on the surface of an identical thermal transfer image-receiving sheet to form a full-color image. The development of various hardwares and softwares associated with multimedia has led to the expansion of the market of the thermal transfer method as a full-color hard copy system for computer graphics, static images through satellite communication, digital images typified, for example, by images of CD-ROMs (compact disc read only memory), and analog images, such as video images.
Specific applications of the thermal transfer image-receiving sheet used in the thermal transfer method are various, and representative examples thereof include proofs of printing, output of images, output of plans and designs, for example, in CAD/CAM, output of various medical analytical instruments and measuring instruments, such as CT scans and endoscope cameras, alternative to instant photographs, output and printing of photograph-like images of a face or the like onto identification cards or ID cards, credit cards, and other cards, and composite photographs and commemorative photographs, for example, in amusement facilities, such as amusement parks, game centers (amusement arcades), museums, and aquaria. The diversification of the applications has lead to an increasing demand for the formation of a thermally transferred image on a desired object. One method proposed for meeting this demand comprises the steps: providing an intermediate transfer recording medium comprising a substrate and a receptive layer separably provided on the substrate; providing a thermal transfer sheet having a dye layer; transferring the dye from the thermal transfer sheet to the receptive layer in the intermediate transfer recording medium to form an image on the receptive layer; and then heating the intermediate transfer recording medium to transfer the receptive layer onto an object (see Japanese Patent Laid-Open No. 238791/1987).
Sublimation transfer-type thermal transfer sheets can faithfully form gradational images, such as photograph-like images of a face. Unlike conventional images produced by printing inks, however, these images disadvantageously lack in fastness properties, such as weathering resistance, abrasion resistance, and chemical resistance. To solve this problem, a method has been proposed wherein a protective layer thermal transfer film having a thermally transferable resin layer is put on top of a thermally transferred image and the transparent thermally transferable resin layer is transferred, for example, by means of a thermal head or heating roll to form a protective layer on the image and thus to protect the image.
The conventional transfer-type protective layer should be partially transferred at the time of transfer by means of a thermal head or a heat roll and thus should have good transferability. In this case, in order to realize good transferability of the protective layer, the protective layer should be a resin layer having a thickness of about several μm. This makes it impossible to impart fastness properties, such as high scratch resistance and chemical resistance, to the protective layer. Further, satisfactory fastness properties, such as scratch resistance and chemical resistance, cannot also be imparted to the protective layer formed on the intermediate transfer recording medium from the viewpoint of transferability. The formation of an image on an object using an intermediate transfer recording medium followed by lamination of a resin film to form a protective layer so as to cover the image on the object is also considered. For some shape of the object, however, the resin film is possibly cockled at the time of lamination, and the process is complicated, for example, due to the necessity of performing processing by means of a specialty machine such as a laminator.
Further, measures for preventing alteration and forgery of the object with an image formed thereon using the intermediate transfer recording medium are unsatisfactory, and, thus, the alteration and forgery cannot be fully prevented.
Accordingly, in order to solve the above problems of the prior art, an object of the present invention is to provide an intermediate transfer recording medium, that can yield thermally transferred images which are excellent in various fastness properties even under severe service conditions, can transfer a protective layer on the image with good accuracy in a simple manner, and can provide an object with an image formed thereon which is highly difficult to be altered or forged, a process for producing the same, and a method for image formation.